1. Field of the Invention
The present invention relates in general to the measurement of acoustic insulation of construction such as building frontages, walls, partitions, anti-noise screens, . . . and more particularly to such measurements when the noise source affecting the structure produces a noise deviating from a reference sound.
A situation of this kind arises frequently in practice, particularly in architectural acoustics, when, for example, the insulating of a building frontage from a reference sound is to be measured, for instance the noise caused by average road traffic. Measurement does not present a particular problem if it is possible to test the system in question directly against the reference sound itself. The problem becomes more complicated when there is no reference sound available to test the system, when for example the building frontage is not subject to the noise of average road traffic.
2. Description of the Prior Art
There are several known devices capable of solving the problem of the direct measurement of an acoustic insulation when the affecting noise deviates from the reference noise by its level, or by its level and its spectral power density if the latter difference is known and does not vary during the measurement. In both cases, the measurement is, in effect, the measurement of the ratio of two quantities, one of the quantities being compared to the other after the spectral correction corresponding to a possible known difference. In all of the other cases, a direct measurement is not possible; two sets of measurements are then required, one enabling the spectral power density of the noise in effect affecting the system to be determined, the other that of the noise in effect transmitted by the system. The results obtained then enable an independent calculation of either the insulation of the system with respect to the reference noise or the level at which this noise is transmitted by the system, the two results being equivalent as their total correspond to the reference noise level. The use of a calculation also enables a possible weighting to be introduced very simply so that a subjective level may be evaluated instead of an objective level. A protocol detailing the measurements and calculations to be carried out, in particular with respect to measuring the insulation of buildings from aircraft noise in relation to noises external to the building, is given in Appendix III Chapter II of the circular No. 72-110 of June 29, 1972 concerning the "Acoustic comfort Label" of the French Ministry of Equipment and Housing and the French Ministry of Transports, sold by the French National Press as a special pamphlet No. 72-54 bis.
If the above operations enable the problem to be solved, they do not enable the result required to be obtained directly and quickly by means of an automatic apparatus.